Suji links tiles three ranks apart (1-4-7, 2-5-8, 3-6-9). It estimates how dangerous a tile is against a riichi. If an opponent discarded a 4, a ryanmen (two-sided) wait on the sides of the 4, that is waiting on 1 or 7, is impossible because of furiten: the 1 and 7 therefore become suji, and safer.
Suji does not make a tile fully safe: depending on its rank it may still complete a kanchan or penchan, and every tile can remain live for tanki or shanpon. A terminal may also complete kokushi musou. Suji only rules out the ryanmen tied to the tile already discarded.
Suji combines with genbutsu (tiles that are absolutely safe because they already sit in the opponent's discards) and reading the kawa to pick the least risky discard. It is the basic tool of defense in riichi mahjong.